Pigments have been proposed, which can be used to mark a substrate by the application of laser energy. Reference can be made to WO-A-00/43456, JP-A-11001065, EP-A0522370, EP-A-0797511, U.S. Pat. Nos. 5,053,440; 5,350,792; 5,928,780; 6,017,972; 6,019,831; 5,489,639 and 5,884,079.
WO02/01250 discloses the use of oxymetal salts in laser marking. AOM (ammonium octamolybdate) is an example of a material that can be marked directly with 10,600 nm laser radiation.
WO05/068207 discloses the use of NIR laser radiation (i.e. at 800 to 2000 nm) to initiate a color change reaction when a NIR-absorbing metal salt is used in combination with a substance that normally undergoes a color change reaction at much longer wavelength (about 10,600 nm), e.g. AOM.
JP8127670 discloses the use of reduced titanium oxide compounds for incorporation into thermoplastics, for the laser marking of molded products.
U.S. Pat. No. 5,911,921 discloses the use of non-stoichiometric ytterbium phosphate to produce NIR absorbing inks, e.g. for printing stealth bar codes.